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Longman Dictionary of Contemporary English
second language
▪ However, it is very likely that articulating a second language interferes with the processing of the first during simultaneous interpreting.
▪ Interference from the native language is probably one of the most noticeable aspects of the early stages in second language learning.
▪ On this basis we may stress the need to revise language teaching methods to come more in line with second language acquisition.
▪ The closer one is to a second language group, either socially or psychologically, the greater the probability of adequate learning.
▪ The hypothesis is that adults have available both ways of developing competence in a second language.
▪ The input hypothesis is fairly revolutionary in second language terms generally.
▪ The relayed message is the source language, while the second language is the target language.
▪ Virtually all the settings in which sign language is learned can also be found in second language learning.
second language

n. A language that is spoken by someone but is not their mother tongue.

Second language

A person's '''second language ''' or L2, is a language that is not the native language of the speaker, but that is used in the locale of that person. In contrast, a foreign language is a language that is learned in an area where that language is not generally spoken. Some languages, often called auxiliary languages, are used primarily as second languages or linguas franca.

More informally, a second language can be said to be any language learned in addition to one's native language, especially in context of second language acquisition, (that is, learning a new foreign language).

A person's first language is not necessarily their dominant language, the one they use most or are most comfortable with. For example, the Canadian census defines first language for its purposes as "the first language learned in childhood and still spoken", recognizing that for some, the earliest language may be lost, a process known as language attrition. This can happen when young children move, with or without their family (because of immigration or international adoption), to a new language environment.